There should be a nameplate welded on the top side of the exhaust bridging the two muffler cans that will say whether it's a GT or a GT2. Depending on the generation (I believe there are 3), this plate will either be on the left side of the exhaust or in the middle (as pictured here: http://www.vividracing.com/catalog/i...gt2catback.jpg)

Unfortunately, to actually read the plate while it's on the car you'll have to fish a camera under there and take a picture of it if you don't want to remove the exhaust to find out.

I have had a couple of these exhausts and I really don't remember it saying what model it was on that label. Their labeling is so poor that I couldn't even tell if the exhaust I had was for an E90 or E92 (even after calling them).